The present invention relates to a decontamination method and system for soil and the like for decontaminating soil of, for example, fields, and water contaminated with radioactive materials reliably and rapidly on site, aiming to perform decontamination with precision and improved efficiency. The Great East Japan Earthquake occurred on March 2011 caused an accident of the Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company. The accident has dispersed harmful radioactive materials in a wide area and contaminated cities and towns, fields, mountains and forests, the sea, lakes and marshes, and rivers. The radioactive materials adhered to or deposited on persons, animals, and plants have endangered their lives and caused serious damage, stopping various industrial activities such as farming, forestry, livestock farming, fishery, and the like.
Removal of such radioactive materials from living environments and industrial activity areas is mandatory for recovery and restart of industrial activities. Decontamination of soil in fields is an urgent problem especially for people engaged in farming.
However, contamination has widely spread over fields and scattered throughout villages and mountains as well as plains. Decontaminating such wide areas with human power requires a great amount of time and labor, which is inefficient. The advanced age of many people engaged in farming also makes the decontamination work extremely difficult.
In order to deal with the contamination treatment or decontamination treatment of such soil, there is a method for decontaminating halide radioactive wastes by dissolving radioactive material into solvent and then separating the radioactive material from the solvent. In this case, halide is dissolved into water, which is a solvent, to precipitate rare earth elements in the solvent, and the precipitate is collected. As a means for separating non-radioactive material from the solvent, the solvent is evaporated or cooled to precipitate the non-radioactive material (refer, for example, to Patent Document 1).
However, in the above decontamination method, the technique in which the radioactive material is collected by dissolving halide in water shows a low collection rate. Further, another technique which includes steps of evaporation and cooling of the solvent requires a heating device and cooling facilities, thereby making the facilities large-scale and expensive.
Still another example of soil decontamination methods includes digging the soil contaminated with harmful chemical materials, putting the soil into a hopper of heating device, and heating the soil while washing by using nitrogen to desorb and separate the contaminants in the soil (refer, for example, to Patent Document 2).
Such a decontamination method also has problems. The method requires time and labor for moving the contaminated soil to a remote decontamination device and additionally for returning the decontaminated soil to the original position. Further, the contaminated soil needs to be excavated deeply, not only the surface soil. Thus, the method requires an appropriate excavating facility, making the decontamination expensive and large-scale. Additionally, the decontamination device further requires a nitrogen washing machine, heating device, and separator, and thus making the decontamination large scale and expensive.
There is still another example of decontamination methods for soil contaminated with radioactive cesium. The contaminated soil is stored in a water supply tank, and carbon-dioxide gas of high partial pressure is injected into the tank to supply hydrogen ions. After extracting cesium ions on the surfaces of soil particles into a liquid phase, the solution is shifted to a separation vessel which is open to air and the carbon-dioxide gas is released into the air. Then, the pH value in the liquid phase is raised to separate therefrom ions such as alkaline earth metals other than cesium and the ions are deposited on carbonates or hydroxides, and the cesium remaining in the liquid phase is condensed and separated (refer, for example, to Non-Patent Document 1).
The above-described soil decontamination method also has shortcomings. In the above decontamination method, the liquid phase of supernatant fluid in the water supply tank is sent to the separation vessel so that the supernatant fluid does not contain much cesium, which has a high specific gravity. This leads to a low efficiency of concentration and separation of cesium. Further, cesium accumulates in the lower part of the water supply tank and promotes attachment and deposition on the soil, and thereby lowering the effect of decontamination. Accordingly, use of decontaminated soil has been difficult and impractical.
Another decontamination method for soil contaminated with radioactive cesium includes adding water to the contaminated soil received in the reaction vessel, placing positive and negative electrodes in the reaction vessel, applying a voltage to the electrodes to attract radioactive cesium ions onto the negative electrode, depositing soil and other matter associated therewith on the positive electrode, and separating and collecting the radioactive cesium from the contaminated soil to significantly reduce the volume of the contaminants (refer, for example, to Non-Patent Document 2).
However, such a decontamination method for soil also has problems. Since the soil is received in the reaction vessel together with other matters, the method requires a high voltage application, which results in poor electrolysis efficiency. The radioactive cesium ions attracted onto the cathode contain foreign matters so that the cesium ions are separated with a low accuracy. Further, since the decontaminated soil also contains other matters, the separation process is time-consuming and the decontaminated soil cannot be used immediately.
To solve such problems, the applicants have developed and proposed a decontamination method and system for soil and the like (refer, for example, to Patent Document 3). The decontamination method and system for soil and the like include the steps of: introducing and dissolving an object to be decontaminated contaminated with radioactive materials into an acid eluting solvent, condensing and separating the radioactive materials from the eluting solvent, wherein the object to be decontaminated includes contaminated soil and contaminated water, one or both of which are collected and introduced into the eluting solvent to perform solid-liquid separation of the radioactive materials and the object to be decontaminated dissolved in the eluting solvent, the soil which has been separated from the eluting solvent is further separated into solid and liquid and collected, aiming to reduce the volume of the contaminated soil and to reutilize the soil which includes no radioactive material and to restart of farming, wherein the eluting solvent in which the radioactive materials after solid-liquid separation are dissolved is electrolyzed and condensed, the radioactive cesium ions deposited on an electrode are adsorbed on an adsorbent and collected, the adsorbent on which the radioactive cesium ions are adsorbed is sealed and stored in the container, and the container is stored in storage facilities as needed, thereby achieving a safe disposal of the radioactive materials.
The applicants had intended to remove radioactive cesium which has a great impact by the proposed decontamination method for soil and the like, and had not intended to decontaminate radioactive materials other than radioactive cesium, for example, tritium (3H) and tritiated water (HTO6).
The tritium, or tritiated hydrogen, is a radioactive isotope of hydrogen with the mass number of three. Tritium is combined with oxygen and exists in water as tritiated water (HTO6). Tritiated water exists in hydrosphere in states of gas, liquid, and solid phases. Tritiated water has been spread widely in vapor, precipitation, groundwater, rivers, lakes, sea water, drink water, and living things.
Generally, tritium has been considered as one of the least toxic radionuclides and taken lightly from a standpoint of effects on living things since tritium taken is distributed uniformly in a body, and tritium has a relatively short biological half-life (2.3 years) and has low energy. Tritium emits low beta rays but the radiation reaches only 1 μm in cells. While circulating through the entire body as blood, the tritium hardly attacks gene DNAs. When tritium is taken into a cell nucleus, the distance between the tritium and DNA becomes closer so that tritium starts attacking DNAs as radioactive cesium does.
Large amounts of hydrogen exits in DNA. Since tritium has the same chemical properties as those of hydrogen, tritium acts normally if replaced by hydrogen.
However, if tritium is changed into helium (He) after emitting radiation, DNAs in the portion where tritium is changed into helium is damaged and then the damage can become a risk, increasing cancer incidence. Such problems have been pointed out.
The background art referred to hereinabove is:    [Patent Document 1] JP-A-10-213697    [Patent Document 2] JP-A-5-192648    [Patent Document 3] JP-A-2014-41066